Wind energy installations, in particular those in the relatively high rating classes, are increasingly being provided with rotors whose rotor blade pitch angles are adjustable. These are referred to as variable pitch rotor blades. In comparison to those with conventional wind energy installations which dominate the relatively low rating classes and have fixed rotor blades, so-called stall-regulated wind energy installations, they offer the advantage that the mechanical power which is extracted from the wind and is fed into the rotor/generator system of the wind energy installation can be influenced by adjusting the rotor blades. Particularly when the windspeed is high, they offer advantages both in terms of efficiency and in terms of control quality in comparison to stall-regulated installations with fixed rotor blades. A further major advantage of wind energy installations with variable pitch rotor blades is that, in the event of a disturbance, the wind energy installation can be slowed down quickly in an emergency by moving the rotor blades to a zero pitch angle position. In the event of disturbances or in the event of a shut-down for safety reasons, for example because of severe stormy winds, this makes it possible to ensure that the rotor behaves like a flag and no longer exerts any torque on the rotor shaft and the generator.
Various systems have become known for adjusting the rotor blades. On the one hand, so-called central pitch devices exist in which the rotor blades are adjusted jointly, generally hydraulically, by means of an actuator which is arranged in the pod of the wind energy installation or in the hub of the rotor. Central pitch devices such as these have the advantage that their control is simple and their design is generally not very complex. Furthermore, it is known for an individual pitch device to be provided to adjust the pitch angle of each individual rotor blade of the rotor. The individual pitch devices are generally designed to act hydraulically or electrically, and are operated by a higher-level central control device for the wind energy installation. In this case, they can be operated in such a way that the rotor blades are operated jointly (collectively) or such that they are moved individually (cyclically) throughout the revolution of the rotor. In addition to the capability for cyclic adjustment, individual pitch devices also offer the advantage that they allow independent adjustment in an emergency as well, in the event of installation component failure. For example, if one individual pitch device fails, the rotor can still be moved essentially to a zero pitch angle position by the others so that at most only a small torque acts on the generator. Wind energy installations such as these are disclosed in DE-A-197 39 164.
One disadvantage of known wind energy installations with adjustable rotor blades is that the emergency adjustment process can result in high mechanical loads. Rapid movement of the rotor blades while the rotor is still running results in a virtually instantaneous change in the longitudinal thrust, which can result in the substructure disadvantageously starting to oscillate. This can be exacerbated in the case of individual pitch devices if the rotor blades are operated at different rates, thus resulting in the possibility of unbalancing and further loads.